Interconnections between the stereovilli of the fish inner ear

Summary The maculae utriculi and sacculi of the inner ear from the European roach (Rutilus rutilus) were investigated by transmission electron microscopy. The stereovilli of peripherally and centrally located sensory cells differ in several features that suggest a developmental gradient. The stereovilli of the peripheral sensory cells, shown to be differentiating cells by other research groups, are short and less steeply graded in height than in central hair cells. All stereovilli in both kinds of hair bundles are interconnected. In the central bundles of stereovilli basal, tip, and vertical connectors are separated by unconnected regions. In contrast, filaments and sometimes other additional structures connect the stereovilli of peripheral bundles over their entire length, but vertical connectors are usually absent. Osmiophilic material occurring inside peripheral stereovilli is interpreted to be monomeric actin. Central and peripheral hair bundles also differ in their reaction to ruthenium red and cationized ferritin. Only the stereovilli of the central cells can be fused by these polycations. Ruthenium red also discriminates between supporting and sensory cells indicating differences in amount or distribution of extracellular material. Hair bundles, intermediate in properties and position between central and peripheral sensory cells, were also found, so that it became possible to propose a scheme of developmental steps leading from microvilli or microvillus-like stereovilli to the fully differentiated hair bundle.     

Integrins and the extracellular matrix: key mediators of development and regeneration of the sensory nervous system

The somatosensory nervous system is responsible for the transmission of a multitude of sensory information from specialized receptors in the periphery to the central nervous system. Sensory afferents can potentially be damaged at several sites: in the peripheral nerve; the dorsal root; or the dorsal columns of the spinal cord; and the success of regeneration depends on the site of injury. The regeneration of peripheral nerve branches following injury is relatively successful compared to central branches. This is largely attributed to the presence of neurotrophic factors and a Schwann cell basement membrane rich in permissive extracellular matrix (ECM) components which promote axonal regeneration in the peripheral nerve. Modulation of the ECM environment and/or neuronal integrins may enhance regenerative potential of sensory neurons following peripheral or central nerve injury or disease. This review describes the interactions between integrins and ECM molecules (particularly the growth supportive ligands, laminin, and fibronectin; and the growth inhibitory chondroitin sulfate proteoglycans (CSPGs)) during development and regeneration of sensory neurons following physical injury or neuropathy.     

Regeneration of sensory axons within the injured spinal cord induced by intraganglionic cAMP elevation

The peripheral branch of primary sensory neurons regenerates after injury, but there is no regeneration when their central branch is severed by spinal cord injury. Here we show that microinjection of a membrane-permeable analog of cAMP in lumbar dorsal root ganglia markedly increases the regeneration of injured central sensory branches. The injured axons regrow into the spinal cord lesion, often traversing the injury site. This result mimics the effect of a conditioning peripheral nerve lesion. We also demonstrate that sensory neurons exposed to cAMP in vivo, when subsequently cultured in vitro, show enhanced growth of neurites and an ability to overcome inhibition by CNS myelin. Thus, stimulating cAMP signaling increases the intrinsic growth capacity of injured sensory axons. This approach may be useful in promoting regeneration after spinal cord injury.     

Nerve growth factor regulates central terminals of primary sensory neurons

Transection of peripheral sensory axons results in transganglionic degenerative atrophy of central terminals of the affected primary sensory neurons. Nerve growth factor applied at the central stump of the transected nerve prevents or delays transganglionic degenerative atrophy. It is concluded that, under normal conditions, nerve growth factor taken up by receptors at peripheral sensory nerve endings and transported retrogradely to perikarya in dorsal root ganglia, regulates synthesis of neuroproteins destined for maintenance of central terminals of these neurons. Accordingly, transganglionic degenerative atrophy is the consequence of failure of nerve growth factor to reach perikarya of primary sensory neurons.     

Early peripheral sensory neurons in the development of trochozoan animals

Neuronal development of the majority of trochozoan animals with biphasic pelago-bentic life cycle starts from transient peripheral neurons, which do not belong to the central nervous system and are mainly located in the apical sensory organ and in the hyposphere. Some of these neurons are pioneer and send neurites that form a scaffold upon which the adult central nervous system later develops. In representative species of molluscs and polychaetes, immunolabelling with the antibodies against neurotransmitters serotonin and FMRFamide, and acetylated α-tubulin revealed that the structure of almost all early peripheral neurons is typical for sensory, most probably chemosensory cells: flask shape, and cilia at the end of the apical dendrite or inside the distal ampoule. Morphology, transmitter specificity, location and projections of the early sensory cells differ in trochophores of different species thus suggesting different origin of these cells. In polychaete larvae, pharmacological inhibition of serotonin synthesis in early peripheral neurons did not affect the development, whereas its increase resulted in developmental arrest and neural malformations, suggesting that early peripheral sensory neurons are involved in developmental regulation.     

Type III neuregulin 1 regulates pathfinding of sensory axons in the developing spinal cord and periphery

Sensory axons must develop appropriate connections with both central and peripheral targets. Whereas the peripheral cues have provided a classic model for neuron survival and guidance, less is known about the central cues or the coordination of central and peripheral connectivity. Here we find that type III Nrg1, in addition to its known effect on neuron survival, regulates axon pathfinding. In type III Nrg1(-/-) mice, death of TrkA(+) nociceptive/thermoreceptive neurons was increased, and could be rescued by Bax elimination. In the Bax and type III Nrg1 double mutants, axon pathfinding abnormalities were seen for TrkA(+) neurons both in cutaneous peripheral targets and in spinal cord central targets. Axon guidance phenotypes in the spinal cord included penetration of axons into ventral regions from which they would normally be repelled by Sema3A. Accordingly, sensory neurons from type III Nrg1(-/-) mice were unresponsive to the repellent effects of Sema3A in vitro, which might account, at least in part, for the central projection phenotype, and demonstrates an effect of type III Nrg1 on guidance cue responsiveness in neurons. Moreover, stimulation of type III Nrg1 back-signaling in cultured sensory neurons was found to regulate axonal levels of the Sema3A receptor neuropilin 1. These results reveal a molecular mechanism whereby type III Nrg1 signaling can regulate the responsiveness of neurons to a guidance cue, and show that type III Nrg1 is required for normal sensory neuron survival and axon pathfinding in both central and peripheral targets.     

Role of motoneuron-derived neurotrophin 3 in survival and axonal projection of sensory neurons during neural circuit formation

Sensory neurons possess the central and peripheral branches and they form unique spinal neural circuits with motoneurons during development. Peripheral branches of sensory axons fasciculate with the motor axons that extend toward the peripheral muscles from the central nervous system (CNS), whereas the central branches of proprioceptive sensory neurons directly innervate motoneurons. Although anatomically well documented, the molecular mechanism underlying sensory-motor interaction during neural circuit formation is not fully understood. To investigate the role of motoneuron on sensory neuron development, we analyzed sensory neuron phenotypes in the dorsal root ganglia (DRG) of Olig2 knockout (KO) mouse embryos, which lack motoneurons. We found an increased number of apoptotic cells in the DRG of Olig2 KO embryos at embryonic day (E) 10.5. Furthermore, abnormal axonal projections of sensory neurons were observed in both the peripheral branches at E10.5 and central branches at E15.5. To understand the motoneuron-derived factor that regulates sensory neuron development, we focused on neurotrophin 3 (Ntf3; NT-3), because Ntf3 and its receptors (Trk) are strongly expressed in motoneurons and sensory neurons, respectively. The significance of motoneuron-derived Ntf3 was analyzed using Ntf3 conditional knockout (cKO) embryos, in which we observed increased apoptosis and abnormal projection of the central branch innervating motoneuron, the phenotypes being apparently comparable with that of Olig2 KO embryos. Taken together, we show that the motoneuron is a functional source of Ntf3 and motoneuron-derived Ntf3 is an essential pre-target neurotrophin for survival and axonal projection of sensory neurons.     

Histochemical localization of galactose-containing glycoconjugates in sensory neurons and their processes in the central and peripheral nervous system of the rat

We studied the distribution of sugar residues in the oligosaccharide chains of complex carbohydrates in tissue sections of rat spinal cord, brainstem, and sensory ganglia using twelve lectin-horseradish peroxidase conjugates. Glycoconjugates containing terminal galactose residues were localized apparently in the Golgi apparatus in a population of predominantly small B-type neurons in spinal and trigeminal ganglia. Large A-type neurons rarely showed reactivity with galactose-binding lectins. A cells stained for glycoconjugates with N-glycosidically linked oligosaccharides and glycogen. The central and peripheral processes of the small neurons, mostly unmyelinated C fibers in sensory roots and spinal nerves, contained an abundance of glycoconjugates with terminal alpha-galactose residues. The central projections and terminals of small to medium-sized primary sensory neurons in the spinal and trigeminal ganglia were visualized in Lissauer's tract and the substantia gelatinosa in the spinal cord, and in the spinal trigeminal tract and the nucleus trigeminus in the lower medulla with lectins specific for terminal alpha-galactose residues. In addition, fibers of the solitary system and the area postrema were reactive with these lectins. The peripheral and central nervous system elements with affinity for galactopyranosyl-specific lectins correspond in distribution with neuroanatomical regions thought to be involved in the transmission and relay of somatic and visceral afferent inputs such as pain and temperature. Such specific localization of a glycosubstance to a distinct subpopulation of neurons and their peripheral and central processes suggests that the particular glycoconjugate may be of physiological significance.     

Cellular distribution of parvalbumin immunoreactivity in the peripheral vestibular system of three rodents

The cellular distribution of parvalbumin immunoreactivity in the vestibular peripheral system of mouse, rat, and guinea pig was investigated by light and electron microscopy. Parvalbumin was found in all neurons of the vestibular ganglia of these species but in the sensory epithelia immunoreactivity was restricted to type I hair cells localized exclusively in the central areas. The very intense staining pattern was similar in the cristae ampullares and utricles of all three species but a faint immunoreaction was also detectable in sensory cells of peripheral areas of rat cristae. The parvalbumin-immunoreactive type I sensory cells are connected by nerve fibres of the calyx unit type which are known selectively to contain calretinin.     

Regeneration of neuron processes of the spinal ganglion after its experimental section in rats

Sensory fibers regeneration after dorsal root ganglion (DRG) section was studied in the rat. After middle cross-section of DRG-13 (left side) there were neurons in the proximal part with damage to peripheral processes and in the distal part with damage to central processes. Axonal ionophoresis of cobalt salts was used for the study of sensory fibers regeneration through the scar in DRG during 3, 7, 15, 30, 120 and 180 days after the damage. Peculiarity regeneration of the sensory fibers was shown in spite of damage localization near ganglion cells body, the regeneration of peripheral and central processes of ganglion cells started already on the 3rd day, and sprouting sensory fibers through the scar of DRG--on the 7th day.     

Mechanisms underlying olfactory neuronal connectivity in Drosophila-the atonal lineage organizes the periphery while sensory neurons and glia pattern the olfactory lobe

Patterning of the antennal lobe of adult Drosophila occurs through a complex interaction between sensory neurons, glia, and central neurons of larval and adult origin. Neurons from the olfactory sense organs are organized into distinct fascicles lined by glial cells. The glia originate from one of the three types of sensory lineages-specified by the proneural gene atonal. Gain-of-function as well as loss-of-function analysis validates a role for cells of the Atonal lineage in the ordered fasciculation of sensory neurons. Upon entry of the antennal nerve to central regions, sensory neurons at first remain closely associated with central glia which lie around the periphery of the lobe anlage. Coincident with the arrival of sensory neurons into the brain, glial precursors undergo mitosis and neural precursors expressing Dachshund appear around the lobe. Sensory neurons and glial cells project into the lobe at around the same time and are likely to coordinate the correct localization of different glomeruli. The influence of sensory neurons on the development of the olfactory lobe could serve to match and lock peripheral and central properties important for the generation of olfactory behavior.     

Organization of the sensory system of the earthworm Lumbricus terrestris (Annelida,Clitellata) visualized by DiI

The anatomical organization of the peripheral and central sensory structures of the earthworm Lumbricus terrestris was investigated applying a fluorescent carbocyanine dye (DiI) as a neuronal tracer. Using whole‐mount preparations and confocal laser scanning microscopy, the pattern of primary sensory cells and pathways of their processes were traced and reconstructed in three‐dimensions. Our study shows that a ventral nerve cord ganglion receives sensory fibers from at least two adjacent segments suggesting that the peripheral nervous system is not segmental in its arrangement and the receptive‐fields of the body wall overlap in earthworms. Furthermore, our result suggests an integrative function of the basiepidermal plexus consists of sensory and motor fibers. J. Morphol., 2012. © 2012 Wiley Periodicals, Inc.     

Identity, origin, and migration of peripheral glial cells in the Drosophila embryo

Glial cells are crucial for the proper development and function of the nervous system. In the Drosophila embryo, the glial cells of the peripheral nervous system are generated both by central neuroblasts and sensory organ precursors. Most peripheral glial cells need to migrate along axonal projections of motor and sensory neurons to reach their final positions in the periphery. Here we studied the spatial and temporal pattern, the identity, the migration, and the origin of all peripheral glial cells in the truncal segments of wildtype embryos. The establishment of individual identities among these cells is reflected by the expression of a combinatorial code of molecular markers. This allows the identification of individual cells in various genetic backgrounds. Furthermore, mutant analysis of two of these marker genes, spalt major and castor, reveal their implication in peripheral glial development. Using confocal 4D microscopy to monitor and follow peripheral glia migration in living embryos, we show that the positioning of most of these cells is predetermined with minor variations, and that the order in which cells migrate into the periphery is almost fixed. By studying their lineages, we uncovered the origin of each of the peripheral glial cells and linked them to identified central and peripheral neural stem cells.     

Somatosensory evoked potentials (SEPs) elicited by magnetic nerve stimulation

Magnetic stimulation of peripheral nerves at distal and proximal sites of the upper and lower extremities and at the midlumbar level were used to elicit cortical somatosensory evoked potentials. Evidence is provided that peripheral nerve trunks, rather than distal receptor afferents, are the anatomical structures stimulated by the electromagnetic fields. Magnetic stimulation of peripheral nerves is considered to be useful for an evaluation of the integrity of proximal nerves, nerve roots and central conduction along sensory pathways. In contrast to electrical nerve stimulation, magnetic stimulation is painless and can be applied to proximal nerves and plexus. By means of proximal nerve stimulation central sensory conduction can be tested even in patients with peripheral nerve lesions or polyneuropathy.     

迷走神经背核的研究进展

迷走神经背核（ＤＭＶ）是一个重要的内脏运动核团和内脏感觉核团。ＤＭＶ与中枢及外周存在广泛的纤维联系。ＤＭＶ和孤束核、最后区一起构成了“迷走感觉运动中枢”。ＤＭＶ存在神经－体液回路,使ＤＭＶ神经元可以直接感受外周血及脑脊液中的信息。ＤＭＶ含乙酰胆碱、儿茶酚胺、神经肽类等多种递质及相应受体。ＤＭＶ参与中枢调节胃肠、心血管及内分泌等生理功能。     

Microscopic localization of cholinesterases in the nervous systems of the lobsters, Panulirus argus and Homarus americanus

Using acetylthiocholine as substrate, microscopically localizable cholinesterase (ChE) activity is demonstrated in neural and glial elements of central and peripheral nervous systems of the lobsters, Panulirus argus and Homarus americanus. Moderate to very intense ChE activity occurs in all synaptic regions of the central ganglia and stomatogastric ganglion, in glial sheaths around neuron somata and peripheral nerve axons, and in cytoplasm of a few nerve cell bodies. Axons, identified as motor, contain extremely little ChE. The principal reaction in peripheral nerves occurs in sheath elements of sensory fibres; in most cases, much of the reaction is lost as the nerves lose the sheaths at the point of entry into brain.     

Persistent larval sensory neurones are required for the normal development of the adult sensory afferent projections in Drosophila

We have tested the hypothesis that larval neurones guide growth of adult sensory axons in Drosophila. We show that ablation of larval sensory neurones causes defects in the central projections of adult sensory neurones. Spiralling axons and ectopic projections indicate failure in axon growth guidance. We show that larval sensory neurones are required for peripheral pathfinding, entry into the CNS and growth guidance within the CNS. Ablation of subsets of neurones shows that larval sensory neurones serve specific guidance roles. Dorsal neurones are required for axon guidance across the midline, whereas lateral neurones are required for posterior growth. We conclude that larval sensory neurones pioneer the assembly of sensory arrays in adults.     

Sensory system evolution at the origin of craniates

The multiple events at the transition from non-craniate invertebrate ancestors to craniates included the gain and/or elaboration of migratory neural crest and neurogenic placodes. These tissues give rise to the peripherally located, bipolar neurons of all non-visual sensory systems. The brain was also elaborated at or about this same time. Were the peripheral and central events simultaneous or sequential? A serial transformation hypothesis postulates that paired eyes and an enlarged brain evolved before the elaboration of migratory neural crest placodal sensory systems. Circumstantial evidence for this scenario is derived from the independent occurrence of the combination of large, paired eyes plus a large, elaborated brain in at least three taxa (cephalochordates, arthropods and craniates) and partly from the exclusivity of the diencephalon for visual system-related distal sensory components versus the restricted distribution of migratory neural crest-placodal sensory systems to the remaining parts of the neuraxis. This scenario accounts for the similarity of all central sensory system pathways due to the primary establishment of descending visual pathways via the diencephalon and midbrain tectum to brainstem motor regions and the subsequent exploitation of the same central beachhead by the migratory neural crest-placodal systems as a template for their organization.     

Mesencephalic root fibers of the trigeminal nerve in the cat

The trigeminal nerve has three motor roots and one sensory root in the cat. One of the motor roots can be divided into two bundles: the larger and the smaller. These motor roots form the common root with the sensory root at the exit from the pons, sometimes being separated partially by the subarachnoidal space between the medial and the ventral part of the common root. The mesencephalic root fibers are observed numerously in all the motor roots. Some degenerated fibers are observed in the sensory root. The transitional zone of the trigeminal nerve root between central and peripheral nervous system is occupied by interlocking processes of the fibrous astrocyte.     

TREK-1 is a heat-activated background K(+) channel

Peripheral and central thermoreceptors are involved in sensing ambient and body temperature, respectively. Specialized cold and warm receptors are present in dorsal root ganglion sensory fibres as well as in the anterior/preoptic hypothalamus. The two-pore domain mechano-gated K(+) channel TREK-1 is highly expressed within these areas. Moreover, TREK-1 is opened gradually and reversibly by heat. A 10 degrees C rise enhances TREK-1 current amplitude by approximately 7-fold. Prostaglandin E2 and cAMP, which are strong sensitizers of peripheral and central thermoreceptors, reverse the thermal opening of TREK-1 via protein kinase A-mediated phosphorylation of Ser333. Expression of TREK-1 in peripheral sensory neurons as well as in central hypothalamic neurons makes this K(+) channel an ideal candidate as a physiological thermoreceptor.